Physics – Plasma Physics
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994jgr....9914863b&link_type=abstract
Journal of Geophysical Research, Volume 99, Issue A8, p. 14863-14876
Physics
Plasma Physics
11
Magnetospheric Physics: Magnetotail, Magnetospheric Physics: Solar Wind/Magnetosphere Interactions, Space Plasma Physics: Kinetic And Mhd Theory, Space Plasma Physics: Numerical Simulation Studies
Scientific paper
Using linear theory and nonlinear MHD simulations, we investigate the resistive and ideal MHD stability of two-dimensional plasma configurations under the isobaric constraint dP/dt=0, which in ideal MHD is equivalent to conserving the pressure function P=P(A), where A denotes the magnetic flux. This constant is satisfied for incompressible modes, such as Alfvén waves, and for systems undergoing energy losses. The linear stability analysis leads to a Schrödinger equation, which can be investigated by standard quantum mechanics procedures. We present an application to a typical stretched magnetotail configuration. For a one-dimensional sheet equilibrium characteristic properties of tearing instability are rediscovered. However, the maximum growth rate scales with the 1/7 power of the resistivity, which implies much faster growth than for the standard tearing mode (assuming that the resistivity is small). The same basic eigenmode is found also for weakly two-dimensional equilibria, even in the ideal MHD limit. In this case the growth rate scales with the 1/4 power of the normal magnetic field. The results of the linear stability analysis are confirmed qualitatively by nonlinear dynamic MHD simulations. These results suggest the interesting possibility that substorm onset, or the thinning in the late growth phase, is caused by the release of a thermodynamic constraint without the (immediate) necessity of releasing the ideal MHD constraint. In the nonlinear regime the resistive and ideal developments differ in that the ideal mode does not lead to neutral line formation without the further release of the ideal MHD constraint; instead of thin current sheet forms. The isobaric constraint is critically discussed. Under perhaps more realistic adiabatic conditions the ideal mode appears to be stable but could be driven by external perturbations and thus generate the thin current sheet in the late growth phase, before a nonideal instability sets in.
Birn Joachim
Hesse Michael
Janicke Lutz
Schindler Karl
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